Interaction and linkage of extension fractures and normal faults: examples from the rift zone of Iceland

Field and photogeological studies were made of 90 zones of interacting fracture segments along the rift zone of Iceland. Each zone consists of a pair of extension fractures or a pair of normal faults, with lengths from metres to kilometres. These zones evolve from an underlapping stage, through an overlapping stage (the most common configuration) and, finally, to a linkage stage. Of all the zones, only 7% are underlapping, whereas 93% are overlapping, with hook-shaped fracture pairs. The length/width ratios of the overlapping zones have a mean value of 3.5. The preferred geometry of the overlapping zones depends upon the initial configuration of the interacting fractures (length, overstep) and the development of the interaction. In the overlapping zones, most fracture pairs show moderate shear (strike-slip) components, related to local variations in the extension (opening) directions. Vertical displacements on normal faults decrease as the overstep and length of overlapping zones increase; both, in turn, are proportional to the total lengths of the faults forming the pair. The geometrical and kinematic features of overlapping spreading centres at mid-ocean ridges show close similarities to those reported here. These similarities indicate that the architecture and evolution of overlapping zones are scale independent.